The semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), complementary metal oxide semiconductor (CMOS) devices, photodiode arrays, charge injection devices and hybrid focal plane arrays, among others.
Solid-state image sensors, also known as imagers, were developed in the late 1960s and early 1970s primarily for television image acquisition, transmission, and display. An imager absorbs incident radiation of a particular wavelength (such as optical photons, x-rays, or the like) and generates an electrical signal corresponding to the absorbed radiation. There are a number of different types of semiconductor-based imagers, including CCDs, photodiode arrays, charge injection devices (CIDs), hybrid focal plane arrays, and CMOS imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems and other image based systems.
These imagers typically consist of an array of pixel cells containing photosensors, where each pixel cell produces a signal corresponding to the intensity of light impinging on that element when an image is focused on the array. These signals may then be used, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image. The photosensors are typically photogates, phototransistors, photoconductors or photodiodes, where the conductivity of the photosensor or the charge stored in a diffusion region corresponds to the intensity of light impinging on the photosensor. The magnitude of the signal produced by each pixel cell, therefore, is proportional to the amount of light impinging on the photosensor.
Active pixel sensor (APS) imaging devices are described in U.S. Pat. No. 5,471,515. These imaging devices include an array of pixel cells, arranged in rows and columns, that convert light energy into electric signals. Each pixel includes a photodetector and one or more active transistors. The transistors typically provide amplification, read-out control and reset control, in addition to producing the electric signal output from the cell.
While CCD technology has a widespread use, CMOS imagers are being increasingly used as low cost imaging devices. A fully compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital imager applications.
A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including a photoconversion device, for example, a photogate, photoconductor, phototransistor, or a photodiode for accumulating photo-generated charge in a portion of the substrate. A readout circuit is connected to each pixel cell and includes at least an output transistor, which receives photogenerated charges from a doped diffusion region and produces an output signal which is periodically read out through a pixel access transistor. The imager may optionally include a transistor for transferring charge from the photoconversion device to the diffusion region or the diffusion region may be directly connected to or part of the photoconversion device. A transistor is also typically provided for resetting the diffusion region to a predetermined charge level before it receives the photoconverted charges.
In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion region accompanied by charge amplification; (4) resetting the floating diffusion region to a known state; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo-charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion region. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower output transistor.
Each pixel cell receives light focused through one or more micro-lenses. Micro-lenses on a CMOS imager help increase optical efficiency and reduce cross talk between pixel cells. A reduction of the size of the pixel cells allows for a greater number of pixel cells to be arranged in a specific pixel cell array, thereby increasing the resolution of the array. In one process for forming micro-lenses, the radius of each micro-lens is correlated to the size of the pixel cell.
The micro-lenses refract incident radiation to the photosensor region, thereby increasing the amount of light reaching the photosensor and thereby increasing the fill factor of the imager. Other uses of micro-lens arrays include intensifying illuminating light on the pixel cells of a non-luminescent display device such as a liquid crystal display device to increase the brightness of the display, display associated with a camera, forming an image to be printed, and as focusing means for coupling a luminescent device or a receptive device to an optical fiber.
One source of image sensor noise is fixed pattern noise (FPN). FPN may manifest as a stationary background pattern in the image which is caused by mismatches in device parameters. FPN is the systematic signal difference between individual pixel cells or groups of pixel cells. FPN can have a variety of physical causes, including small local variations above each photosensor, differences in electronic response, and variations in the thin film stack above each photosensor, including variations of the color filter and micro-lens layers. FPN in a image sensor is typically around 1.0 to 1.2%, thus the signal to noise ratio due to FPN is about 40 dB.
There is needed, therefore, imaging devices for sensing objects which have reduced fixed pattern noise. A reduction of FPN of about 10 to 20% would improve the image quality that can be sensed by the eye. A 1-2% change in optical transmission, either for the full operating range of wavelengths or a portion thereof would result in an appreciable difference in the quality of the image. A method of reducing the fixed pattern noise of the imaging device and methods for fabricating the devices having reduced fixed pattern noise are also needed.